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Home > 9. Operational/Synoptic > 9.27 MODIS Optics

9.27 Visualizing Satellite-Measured Optical Properties and Pigments in IDV: MODIS-A via THREDDS

  • Exercise Title:  Visualizing Satellite-Measured Optical Properties and Pigments in IDV: MODIS-A via THREDDS

  • Abstract:  In this exercise you'll learn the most direct way to obtain and visualize recent MODIS-A optical properties data (in this case chlorophyll-a.  This method follows on other THREDDS-dependent exercises that are replacing more laborious methods that formerly required downloading desired data files.

  • Preliminary Reading (in OceanTeacher, unless otherwise indicated):

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  March 2013

IMPORTANT NEW INFORMATION:  Recently the Saga GIS software has been shown to deal quite adequately with most NetCDF (NC) data grids (and possibly also GRIB files).  This should be viewed as a useful addition to the rapid, direct NC analysis and display capabilities of IDV (see below).  If you are also interested in this route for your data, then please check Viewing and Managing Well-Formed NetCDF Grids in Saga.
1.  Run IDV.
2.  Navigate to this data source:  IDV > DASHBOARD > DATA CHOOSERS > CATALOGS > IDV CATALOG > NOAA OCEANOGRAPHY DATA > OCEAN WATCH > SATELLITE DATASETS > COLOR > Chlorophyll-a, Aqua MODIS, NPP, Global, Science Quality
3.  Within the available chlorophyll products, the 8-day product is probably the most attractive, because it is near-near-real-time, but contains enough data to show something useful.  Click on ADD SOURCE at the bottom of the window.

NOTE:  In the mid-latitudes daily data are often almost completely obscured by clouds.  You'll have to make judgements about the "best" aggregation period for your data type and area of interest.

4.  After the THREDDS server catalog has been examined, you'll see this listing of the data you selected.
5.  Right-click on the "8-day" data object in the DATA SOURCES panel, and select PROPERTIES.
6.  We will only need the TIMES and SPATIAL SUBSET tabs to select the specific data we need.

Select TIMES > USE SELECTED to allow the selection of specific times.

Scroll down to the very last data object, and select it.


7.  Select SPATIAL SUBSET.  Then use your cursor to draw a small rectangle anywhere on the map. 
8.  Now ou can enter the correct coordinates of your area of interest.  [The box on the map will not change while you do this.]

To complete the subsetting, click on OK.

10.  You'll see this draft map first.  It does not show anything, because IDV has selected a bad default value range and a bad color palette.

NOTE:  "Bad" means the author doesn't like them.

11.  On the dashboard, select COLOR TABLE > DEFAULT > RADAR > DBZ, to get a better, rainbow-type palette.
12.  On the dashboard, also select COLOR TABLE > DBZ > CHANGE RANGE
13.  In the small window that appears, enter more reasonable values of chlorophyll.  [For example, 98% of the open sea is less than 2 mg m-3.]

Then click OK.

14.  Now you can see more detail, with a color palette that is easier to understand.  Look closely and you can easily see areas of very high (relatively) values along shore, and broad regions of higher values extending east-west above and below the equator (probably related to the zonal wind patterns.
15.  This analysis can be save with FILE > SAVE AS > PRODUCTS > IDV with the filename chloro_a_liberia_201303110_modis_a_noaa_oceanwatch.xidv.  You'll find much more to do with optical data from this source, which should be explored on your own time.